1. Field of the Invention
The present invention relates to a planetary roller type power transmission device having a first rotary shaft and a second rotary shaft which are arranged coaxially, a sun roller connected to the first rotary shaft, a stationary outer ring disposed concentrically with the sun roller, and a plurality of planet rollers disposed in the annular space between the sun roller and the stationary outer ring, the planet rollers being carried by a carrier rotatable as a unit with the second rotary shaft, whereby a torque is transmitted between the first and the second rotary shafts through the planet rollers.
2. Description of the Prior Arts
In a planetary roller type power transmission device of the type described, it is quite important to maintain adequate levels of contact pressure between the sun roller and the planet rollers and between the planet rollers and the stationary ring, in order to attain smooth operation by eliminating slippage and play between these parts. One of the practical methods of attaining adequate levels of contact pressure is to suitably select the outside diameter of the sun roller, outside diameter of the planet rollers and the inside diameter of the stationary ring so that the desired levels of contact pressure are obtained when these parts are assembled. This method, however, requires the sun roller, planet rollers, and the stationary ring to be finished with extremely high precision. In addition, since there is no means for adjusting the contact pressure, the level of contact pressure is gradually decreased due to wear of the sun roller, planet rollers, and the stationary ring during the use of the power transmission device.
Japanese Patent Laid-Open No. 111049/1979 (Laid-Open on Aug. 31, 1979) discloses a planetary roller type power transmission device improved to overcome the above-described problems of the conventional power transmission device. This power transmission device has a stationary outer ring provided with a pair of flange-like projections which are opposite each other and extend in circumferential directions; and a casing comprised of a peripheral wall portion surrounding the stationary outer ring and an end wall portion contacting one axial end surface of the peripheral wall portion and secured to the peripheral wall portion by bolts. The peripheral wall portion has a projecting wall which projects radially inward, while the end wall portion has a projected wall which projects inwardly in the axial direction. When the peripheral wall portion of the casing and the end wall portion of the casing are assembled, the projected wall of the peripheral wall portion is made to contact the outer surface of one of the flange-like projections, while the projected wall of the end wall portion is made to contact the outer surface of the other flange-like projection so that both projected walls exert a force which urges the pair of flange-like projections towards each other, thus causing radially inward elastic deformation of the stationary outer ring to produce the desired levels of contact pressure between the sun roller and the planet rollers and planet rollers and the stationary ring. This power transmission device, however, still suffers from the disadvantage that, since the relative position between the peripheral wall portion and the end wall portion of the casing and hence the relative position between both projected walls are unchangeable, it is difficult to adjust the contact pressure by adjusting the force which is applied by the projected walls to the pair of flange-like projections. To deal with this problem, a spacer or shims are placed between the projected wall of the end wall portion and the outer surface of the cooperating flange-like projection, as a means for adjusting the above-mentioned force. In this power transmission device, when adjustment of the contact pressure becomes necessary after assembly, it is necessary to replace the spacer with another spacer of different thickness or vary the number of the shims after detaching the end wall portion from the peripheral wall portion of the casing.